It is frequently desirable, e.g. for the detection of malfunctions or for traffic surveys, to identify the units of different groups which are temporarily associated in a given connection. A possible way of accomplishing this, which may be described as a deterministic method, involves the introduction of a signaling criterion into the first unit called into service and the subsequent tracing of that criterion in all the other units engaged in the connection. Such a monitoring system, using for example individual sensors for all the units involved, is relatively complex and costly.